A nuclear fuel assembly that is at its end period of a nuclear fuel cycle, finishes its combustion, and cannot be used any more, is called spent nuclear fuel. The spent nuclear fuel containing high radioactive materials such as FP requires thermal cooling, and is, therefore, cooled at a cooling pit of a nuclear power plant for a predetermined period of time (one to three years). Then, the spent nuclear fuel is accommodated in a cask as a shielding container, and the cask is conveyed to a reprocessing facility by track or the like, and is stored there. Each spent fuel assembly is inserted into each cell of a basket installed in the cask, thereby to secure proper holding force against vibration during the transportation.
There are various kinds of casks disclosed as conventional examples in “Nuclear Eye (in Japanese), Nikkan Kogyo Syuppan Production, issued on Apr. 1, 1998, and Japanese Patent Application Laid-open No. 62-242725. A cask that has been helpful for developing the present invention will be explained below. The following contents will be shown only for the sake of convenience of explanation, and do not mean known or public contents.
FIG. 39 is a perspective view which shows one example of a cask. FIG. 40 is a cross-sectional view of the cask shown in FIG. 39 in its axial direction. The cask 500 is constructed of a cylindrical body 501, a resin 502 as a neutron shield provided on the external periphery of the body 501, external cylinder 503 thereof, a bottom section 504, and a lid section 505. The body 501 and the bottom section 504 are made of carbon steel as a gamma-ray shield by forging. The lid section 505 consists of a primary lid 506 and a secondary lid made of stainless steel or the like. The body 501 and the bottom section 504 are connected together by butt welding. The primary lid 506 and the secondary lid 507 are fixed to the body 501 with bolts made of stainless steel or the like. A metallic O-Ring is provided between the lid section 505 and the body 501, thereby to keep the internal side of the body airtight.
A plurality of heat conductive fins 508 are provided between the body 501 and the external cylinder 503 to carry out thermal conduction. In order to increase thermal conductivity, copper is used as the material of the heat conductive fins 508. The resin 502 is injected into a space formed between the heat conductive fins 508 in a fluid status, and is cooled and fixed. A basket 509 has a structure having 69 angular pipes 510 collected in a flux shape as shown in FIG. 26, and is inserted into a cavity 511 of the body 501 so as to be restrained within the cavity 511.
The angular pipes 510 are made of aluminum alloy mixed with a neutron absorber (boron: B) to avoid the inserted spent fuel assemblies from reaching a critical state. At both sides of a body 512 of a cask, trunnions 513 are provided (one is omitted) to suspend the cask 500. Further, buffers 514 having wood built inside as a buffer material are fitted (one is omitted) at both ends of the cask body 512.
In the cask 500, the heat conductive fins 508 are welded to the body 501, the external cylinder 503 is disposed around the fins, and welding is carried out from the inside of the external cylinder. However, since the cask 500 has a height of about 6 m and the space formed between the external cylinder 503 and the heat conductive fins 508 is extremely narrow and long, it is extremely difficult to manually carry out welding. Therefore, in the current status, an exclusive self-run welding machine is manufactured, and the welding machine must be run in the space to carry out welding by remote control. Consequently, there has been a problem that the welding operation is troublesome. Particularly, when only a traditional welding machine exists in an assembling plant, it is difficult to assemble the cask. As the introduction of an exclusive welding machine and a technical guidance become necessary, the assembling can be carried out at only a specific assembling plant. This problem becomes serious in foreign countries where the assembling technique has not been developed.
This invention has been achieved in order to solve the above problem, and it is a first object of the invention to provide a cask whose assembly is easily carried out and a method of manufacturing the cask.
Next problems of the conventional cask are as follows. FIG. 41 is a structural diagram which shows one example of the conventional cask. FIG. 42 is a cross-sectional view of the cask shown in FIG. 41 in its axial direction. In this cask 1500, a spent fuel assembly is accommodated in a stainless body 1501 that shields the gamma ray (not shown) Around the external periphery of the body 1501, a plurality of stripes 1502 are welded in a radial shape. These stripes 1502 have their cross sections bent in an approximately v shape, and have their side edges 1521 welded to the body 1501, with the other side edges 1522 welded to bent lines 1523 formed at corners of adjacent stripes 1502. In order to discharge decay heat conducted from the body 1501, the stripes 1502 are constructed of a material having good thermal conductivity, for example, copper plates or the like. A space formed between the stripes 1502 is filled with a resin 1503 to absorb neutron.
A fin 1504 for heat radiation is welded to the outside of each stripe 1502. This fin 1504 has a comb shape and has its tooth surface formed in a curvature. The decay heat of the spent fuel assemblies is first transmitted to the body 1501. As the stripes 1502 have been welded to this body 1501, the decay heat is conducted from this body 1501 to the stripes 1502, and is then discharged to the outside through the externally exposed surface of the stripes 1502 and the fins 1504.
However, according to the above conventional cask 1500, as the stripes 1502 have the fins 1504 welded, there has been a problem that the temperature is distributed over the external surface, and it is not possible to radiate heat uniformly. In other words, the temperature distribution occurs in such a manner that portions of junction with the fins 1504 have a high temperature, and the stripes 1502 between the fin 1504 and the fin 1504 have a low temperature. This has not been desirable in terms of heat radiation efficiency. This invention has been made in the light of the above aspect, and it is a second object of the invention to provide a cask capable of improving the heat radiation efficiency.